We've got a placeholder for collection icons in place as of bug 487397, but a real end-to-end feature needs to be implemented for:
* accepting image uploads for collection icons
* hosting and serving uploaded collection icon images
We could stuff these into the database as blobs like addon icons, but this might be a good time to look at an alternative approach. (eg. CDN, or at least a file system behind a static apache server somewhere.)

Committed to r24626 (test framework) and r24627 (code) in order to un-block bug 456132. The tests pass, but please feel free to point out any issues you may have with this code or any changes you require.
I also added the SQL to the wiki page.